In a WDM optical transmission system applied to long-distance trunk lines, optical amplification repeaters are connected in multiple stages in order to achieve low-cost and low-noise optical transmission. Each of the optical amplification repeaters usually includes a simple optical amplifier, and generally does not include a wavelength-deviation compensator capable of independently adjusting a power level corresponding to signal light of each wavelength (a channel) included in WDM light. Accordingly, level deviations among channels that occur in the WDM light in accordance with gain-wavelength characteristics of the optical amplification repeaters are accumulated in accordance with the number of stages of the optical amplification repeaters. Accordingly, a phenomenon occurs in which an optical signal-to-noise ratio (OSNR) deteriorates on a channel having a relatively low power level among a plurality of channels of the WDM light that reaches a receiving end. In this regard, a power level of a received optical signal is sometimes called a “reception level”. Also, an OSNR of a received optical signal is sometimes called a “received OSNR”.
FIG. 1 is a schematic diagram illustrating a state in which level deviations of the above-described WDM light among channels are accumulated in accordance with the number of repeated stages. An upper side of FIG. 1 illustrates an example of a configuration of a trunk-line WDM optical transmission system in which WDM light output from a transmission terminal station 101 is transmitted to a receiving terminal station 102 while being amplified by a plurality of optical amplification repeaters 104 disposed on an optical transmission line 103 at necessary intervals. Also, a lower side of FIG. 1 illustrates level deviations among channels of WDM light at each position along that system.
In FIG. 1, it is assumed that each optical amplification repeater 104 disposed between the transmission terminal station 101 and the receiving terminal station 102 is constructed using a simple optical amplifier which does not have a function of an OADM (Optical Add Drop Multiplexer) node that adds or drops a specific channel to or from WDM light transmitted on the optical transmission line 103, and a function that can independently adjust a power level corresponding to each channel. In this case, WDM light with a same level of each channel power, which is output from the transmission terminal station 101, is amplified by a first-stage optical amplification repeater 104 so that differences arise in each channel power depending on individual gain-wavelength characteristics. Accordingly, WDM light given at an input end al of an optical transmission line 103 of a first repeater span connected to the first-stage optical amplification repeater 104 has a wavelength deviation corresponding to a difference between a maximum power level corresponding to a channel indicated by a broken line and a minimum power level corresponding to a channel indicated by a chain-dotted line in the lower side of FIG. 1. In this regard, a bold solid line in the lower side of FIG. 1 denotes an average power level of individual channels of the WDM light.
The WDM light transmitted to the first repeater span reaches an output end b1 of the first repeater span while being attenuated in accordance with a loss characteristic of the optical transmission line 103, and is amplified by the second-stage optical amplification repeater 104. In the second-stage optical amplification repeater 104, differences occur in power of individual channels in accordance with the gain-wavelength characteristic in the same manner as the case of the first-stage optical amplification repeater 104. Accordingly, WDM light given at an input end a2 of a second repeater-span optical transmission line 103 has a wavelength deviation produced by accumulating the wavelength deviations individually occurring in the first-stage and the second-stage optical amplification repeaters 104. In the same manner as described above, wavelength deviations that occur by individual optical amplification repeaters 104 of a third stage and thereafter are accumulated in sequence so that WDM light that reaches the receiving terminal station 102 has enlarged level deviations among channels as shown by the right end of the lower part of FIG. 1. Among a plurality of channels included in the WDM light, for channels that have a relatively low power level, a ratio of the signal component to the noise component, which is accumulated ASE, etc., that occurred in the individual optical amplification repeaters 104, is decreased, and thus the above-described deterioration of OSNR becomes problematic.
A related-art technique for suppressing deterioration of an OSNR caused by accumulation of level deviations of WDM light among channels as described above is described. For example, a technique is provided for performing pre-emphasis on the basis of measurement results of the OSNR as described in a patent document, Japanese Laid-open Patent Publication No. 8-321824, etc. Specifically, as shown in FIG. 2, in the above-described technique, an OSNR monitor 105 is disposed at a receiving terminal station 102, and a power level of each channel transmitted from a transmission terminal station 101 is optimized by a pre-emphasis control circuit 106 so as to maximize an OSNR of each channel measured by the OSNR monitor 105.
However, the above-described related-art technique has a drawback in that it becomes necessary to provide an expensive device, such as an optical spectrum analyzer, etc., as the OSNR monitor 105. Also, as shown by a dotted line in FIG. 2, in the case of a system configuration in which an OADM node 107 is disposed on an optical transmission line 103 between the transmission terminal station 101 and the receiving terminal station 102, a lot of transmission and reception sections (wavelength paths) corresponding to each channel of WDM light may be provided. In this case, if pre-emphasis is to be optimized for each of the transmission and reception sections, there is the possibility that an optimum solution thereof cannot be found.
On the problem of the related-art technique described above, for example, a patent document, Japanese Laid-open Patent Publication No. 2001-203414 has proposed a technique in which predetermined physical quantities at a plurality of stations on an optical transmission line are obtained in place of measuring a level of amplified spontaneous emission (ASE) noise included in WDM light using an optical spectrum analyzer, etc., an OSNR corresponding to each channel at a receiving end is calculated on the basis of the physical quantities, and pre-emphasis is performed so that the OSNR at each of the channels becomes equal with each other. In this technique, when an OADM node is disposed between the transmission end and the receiving end, it is made possible to optimize pre-emphasis corresponding to each channel of WDM light by making path groups which gather channels having same transmission/receiving sections, performing pre-emphasis among the individual path groups, and then performing pre-emphasis for each of the path groups.
However, in the above-described related-art technique, if the number of OADM nodes disposed between the transmission end and the receiving end increases, the transmission/receiving sections corresponding to individual channels of WDM light have a large variety of paths. Accordingly, calculation of OSNRs corresponding to the individual transmission/receiving sections and adjustment of transmission light level become complicated, and thus there is a problem in that it is not easy to achieve the technique. Also, if pre-emphasis is performed so that an OSNR of each channel at a receiving end becomes equal, level differences among channels at the transmission end sometimes become considerably large. It is possible that such a large wavelength deviation of WDM light at transmission end has an adverse effect on transmission characteristics other than the OSNR.